The present invention relates to a photodetecting device for receiving light incident on a side surface of a semiconductor substrate. More particularly, it relates to a photodetecting device of side-surface illuminated type wherein a gradient surface for refracting or reflecting incident light is formed in a second principal surface in opposing relation to a first principal surface on which a light-receiving portion is formed, thereby changing the optical path of the incident light.
As a photodetecting device for fiber-optics communications that is sensitive to light in a long wavelength band of approximately 1.3 to 1.55 xcexcm, there has widely been used a pin photodiode using an InGaAs/InP compound semiconductor as a material.
Typical pin photodiodes include a top-surface illuminated type for receiving light at the light-receiving portion side and a back-surface illuminated type for receiving light at the side opposite to the light-receiving portion, which are used selectively depending on the direction of connection to an optical fiber.
In recent years, there has been developed a photodetecting device for receiving light incident on a side surface of a semiconductor substrate. The photodetecting device of side-surface illuminated type is useful in the case where an optical fiber is attached to the photodetecting device in parallel relation to the mount surface of a flat package to which the photodetecting device has been bonded or where the photodetecting device is used to monitor light emitted backward from a semiconductor laser diode that has been bonded to the same mount surface as the photodetecting device.
As examples of the conventional photodetecting device of side-surface illuminated type, pin photodiodes each internally provided with means for changing an optical path by refraction or reflection will be described with reference to the drawings. FIGS. 13(a) and 13(b) show cross-sectional structures of the conventional pin photodiodes disclosed in Japanese Patent Application Laid-Open Publication No. 8-316506. As shown in FIG. 13(a), a buffer layer 102 composed of n-type InP, a light absorbing layer 103 composed of intentionally undoped n-type InGaAs, and a window layer 104 composed of intentionally undoped n-type InP are formed sequentially on a first principal surface 101a of a semiconductor substrate 101 composed of n-type InP.
A p-type impurity such as Zn has been diffused into the window layer 104 to form an island pattern, in which first and second diffused regions 104a and 104b are formed at a specified interval. The portion of the light absorbing layer 103 underlying the first diffused region 104a serves as a light-receiving region 103a. What results is a pin junction formed by the p-type first diffused region 104a, the intentionally undoped n-type light-receiving region 103a, and the n-type buffer layer 102.
A cathode 105 is formed on the first diffused region 104a, while an anode 106 is formed on the second diffused region 104b. 
A second principal surface 101b of the semiconductor substrate 101 in opposite relation to the first principal surface 101a is formed with gradient portions 101c having exposed surfaces located at the side edge portions of the semiconductor substrate 101. If light 201 is incident on the gradient portion 101c in 10 parallel to the second principal surface 101b, the incident light 201 is refracted by the gradient portion 101c before reaching the light-receiving region 103.
Thus, in the conventional photodetecting device, the gradient portion 101b provided in the second principal surface 101b refracts the light incident thereon in parallel to the second principal surface 101b and thereby changes the optical path of the incident light. According to the foregoing publication, a (111) plane orientation is used preferably such that an angle of 54.7xc2x0 is formed between the gradient portion 101c and the second principal surface 101b. This is because the gradient portion 101c is required conditionally to form a specified angle with respect to the second principal surface 101b and have a flat and smooth surface.
To provide the semiconductor substrate with such a gradient portion 101c as to form a specified angle and have a flat and smooth surface (mirrored surface), it is the easiest to perform wet chancel etching whereby a specified crystal plane orientation is exposed. In manufacturing a photodetecting device, a semiconductor substrate using a (001) plane at a principal surface is normally employed so that, when wet chemical etching for exposing a crystal plane orientation is performed, a (111) plane is exposed in most cases.
If it is assumed that the same components as shown in FIG. 13(a) are designated by similar reference numerals in FIG. 13(b), the second principal surface 101b of the semiconductor substrate 101 is formed with gradient portions 101d having exposed surfaces located at the near center portion thereof. In this case, the gradient portion 101d provided in the second principal surface 101b reflects light incident thereon in parallel to the second principal surface 101b and thereby changes the optical path of the incident light. The foregoing publication suggests the use of a (111) plane also at the gradient portion 101d. 
However, the aforesaid conventional photodetecting device of side-surface illuminated type has the problem that the device is larger in chip size than the photodetecting device of top-surface or back-surface illuminated type.
Specifically, if a (111) plane forming an angle of 54.7xc2x0 with respect to the second principal surface 101b is used at the gradient portion 101c of the photodetecting device shown in FIG. 13(a), the angle formed between the light 202 incident on the side edge portion and the second principal surface 101b in the semiconductor substrate 101 becomes 25.7xc2x0. If the thickness of the semiconductor substrate 101 is assumed to be 200 xcexcm, the incident light 201 should travel 416 xcexcm in a direction parallel to the principal surface to reach the first principal surface 101a. 
This indicates that a distance of 416 xcexcm is necessary between the plane of incidence and the center of the light-receiving region 103a regardless of the largeness of the light-receiving region 103. The distance is extremely large considering that the typical chip size of a photodetecting device having a light-receiving region with a diameter of 80 xcexcm is 300 xcexcm square (the distance between the end face of incidence and the center of the light-receiving region is 150 xcexcm) and that the chip size of a photodetecting device having a light-receiving region with a diameter of 300 xcexcm is approximately 500 xcexcm square (the distance between the end face of incidence and the center of the light-receiving region is 250 xcexcm).
On the other hand, the direction of travel of the incident light 201 reflected by the gradient portion 101d is tilted by 19.4xc2x0 from a normal to the second principal surface 101b in the photodetecting device shown in FIG. 13(b). If the thickness of the semiconductor substrate 101 is assumed to be 200 xcexcm, the distance traveled by the incident light 201 in a direction parallel to the principal surfaces till it reaches the first principal surface 101a is 70 xcexcm.
According to the foregoing publication, a pattern provided on the mount on which the photodetecting device is to be mounted is aligned with the end face of the semiconductor substrate 101 formed with the photodetecting device. However, since the distance between the end face of the semiconductor substrate 101 and the light-receiving region 103a is determined not by the accuracy of photolithography, but by the accuracy of dicing, it is difficult to enhance the accuracy. This leads to the problems that the position at which the light is incident is controlled less accurately and that the efficiency with which the incident light 201 is optically coupled to the light-receiving region 103a is lowered.
In the photodetecting device of side-surface illuminated type or back-face illuminated type, in particular, electron-hole pairs are generated when light is incident on the region of the light absorbing layer 104 other than the light-receiving region 103a. Since no electric field is present in this region, the generated holes are moved by diffusion for a long period of time to eventually reach the first diffused region 104a, which causes the problem that a tail current resulting from such a photoelectric current with low responsivity is likely to occur.
It is therefore a first object of the present invention to reduce the size of a photodetecting device of side-surface illuminated type by solving the conventional problems mentioned above. A second object of the present invention is to accurately control the position at which light is incident. A third object of the present invention is to suppress a tail current.
A first method of manufacturing a photodetecting device according to the present invention comprises: a light-receiving portion forming step of forming a light-receiving portion on a first principal surface of a semiconductor substrate; a mask pattern forming step of forming a mask pattern on a second principal surface of the semiconductor substrate in opposing relation to the first principal surface; and an etching step of performing etching with respect to the second principal surface by using the mask pattern to form a gradient portion forming an angle of approximately 35xc2x0 with respect to the second principal surface.
In accordance with the first method of manufacturing a photodetecting device, the second principal surface in opposing relation to the first principal surface of the semiconductor substrate having the light-receiving portion formed thereon is provided with the gradient portion forming an angle of approximately 35xc2x0 with respect to the second principal surface. Accordingly, the angle formed between the light incident on the side portion of the semiconductor substrate and the second principal surface in the semiconductor substrate becomes 41.0xc2x0. If the thickness of the semiconductor substrate is assumed to be 200 xcexcm, the distance traveled by the incident light in a direction parallel to the principal surface till it reaches the first principal surface becomes 230 xcexcm, so that the distance traveled by the incident light in a direction parallel to the principal surface is reduced compared with the conventional case where the distance traveled is 416 xcexcm and the gradient portion is at 54.7xc2x0. This reduces the chip size in a direction parallel to the principal surface.
In the first method of manufacturing a photodetecting device, the semiconductor substrate is preferably composed of indium phosphide, the second principal surface preferably has a (001) plane orientation, the mask pattern forming step preferably includes orienting an aperture of the mask pattern in a near [xe2x88x92110] direction, and the etching step preferably includes performing wet chemical etching using a solution mixture containing hydrochloric acid and nitric acid at a volume ratio of approximately 5:1 to 3:1. This ensures the provision of a mirrored (112) plane forming an angle of 35.3 with respect to the second principal surface as the plane orientation of the gradient portion.
In the present application, the sign xe2x80x9cxe2x88x92xe2x80x9d leading an index indicative of a crystal plane orientation or zone axis represents the inversion of the index following the sign xe2x80x9cxe2x88x92xe2x80x9d.
A second method of manufacturing a photodetecting device according to the present invention comprises: a light-receiving portion forming step of forming a light-receiving portion on a first principal surface of a semiconductor substrate; a mask pattern forming step of forming a mask pattern on a second principal surface of the semiconductor substrate in opposing relation to the first principal surface; and an etching step of performing etching with respect to the second principal surface by using the mask pattern to form a gradient portion forming an angle of approximately 45xc2x0 with respect to the second principal surface.
In accordance with the second method of manufacturing a photodetecting device, the second principal surface in opposing relation to the first principal surface of the semiconductor substrate having the light-receiving portion formed thereon is provided with the gradient portion forming an angle of approximately 45xc2x0 with respect to the second principal surface. Accordingly, the angle formed between the light incident on the side portion of the semiconductor substrate and the second principal surface in the semiconductor substrate becomes 33.2xc2x0. If the thickness of the semiconductor substrate is assumed to be 200 xcexcm, the distance traveled by the incident light in a direction parallel to the principal surface till it reaches the first principal surface becomes 306 xcexcm, so that the distance traveled by the incident light in a direction parallel to the principal surface is reduced compared with the conventional case where the distance traveled is 416 xcexcm and the gradient portion is at 54.7xc2x0. This reduces the chip size in a direction parallel to the principal surface.
In the second method of manufacturing a photodetecting device, the semiconductor substrate is preferably composed of indium phosphide, the second principal surface preferably has a (001) plane orientation, the mask pattern forming step preferably includes orienting an aperture of the mask pattern in a near [xe2x88x92110] direction, and the etching step preferably includes performing wet chemical etching using a solution mixture containing hydrochloric acid, acetic acid, and hydrogen peroxide. This ensures the provision of a mirrored (112) plane forming an angle of 45xc2x0 with respect to the second principal surface as the plan orientation of the gradient portion.
A first photodetecting device according to the present invention comprises: a semiconductor substrate; and a light-receiving portion formed on a first principal surface of the semiconductor substrate, the semiconductor substrate having a gradient portion exposed by partially removing a second principal surface in opposing relation to the first principal surface, the gradient portion having an exposed surface forming an angle of approximately 35xc2x0 to 45xc2x0 with respect to the second principal surface, the light-receiving portion receiving light incident on a side portion of the semiconductor substrate and refracted or reflected by the gradient portion.
In the first photodetecting device, the second principal surface in opposing relation to the first principal surface of the semiconductor substrate having the light-receiving portion formed thereon is provided with the gradient portion forming an angle of approximately 350xc2x0 to 45xc2x0 with respect to the second principal surface. Accordingly, the angle formed between the light incident on the side portion of the semiconductor substrate and the second principal surface in the semiconductor substrate invariably becomes larger than 54.7xc2x0, which is the angle formed between the incident light and the second principal surface in the conventional photodetecting device. This achieves a reduced distance between the plane of incidence and the center portion of the light-receiving region as well as a reduced chip size in a direction parallel to the principal surface.
In the first photodetecting device, the exposed surface of the gradient portion is preferably located at the side portion of the semiconductor substrate. In the arrangement, the light coming from the side of the semiconductor substrate can be projected directly on the gradient portion and refracted thereby.
In the first photodetecting device, the exposed surface of the gradient portion is preferably located at a near center portion of the second principal surface of the semiconductor substrate.
In the arrangement, if the exposed surface of the gradient portion forms an angle of approximately 35xc2x0 with respect to the second principal surface, the direction of travel of the incident light reflected by the gradient portion is tilted by 19.4xc2x0 from a normal to the second principal surface so that the reflected light travels further away from the plane of incidence in contrast to the conventional case where the reflected light travels backward approaching to the plane of incidence. Therefore, the light can be incident on either side portion of the semiconductor substrate.
In the first photodetecting device, the second principal surface preferably has a (001) plane orientation and the exposed surface of the gradient portion preferably has a (112) plane orientation. In the arrangement, the angle formed between the second principal surface and the exposed surface of the gradient portion invariably becomes 35.3xc2x0.
In the first photodetecting device, the second principal surface preferably has a (001) plane orientation and the exposed surface of the gradient portion preferably has a (101) plane orientation. In the arrangement, the angle formed between the second principal surface and the exposed surface of the gradient portion invariably becomes 45xc2x0.
A second photodetecting device according to the present invention comprises: a semiconductor substrate; and a ligh-treceiving portion formed on a first principal surface of the semiconductor substrate, the semiconductor substrate having a side gradient portion exposed by partially removing a second principal surface in opposing relation to the first principal surface and a center gradient portion, the side gradient portion having an exposed surface located at a side portion of the second principal surface, the center gradient portion having an exposed surface located at a near center portion of the second principal surface, the light-receiving portion receiving light incident on a side portion of the semiconductor substrate, refracted by the side gradient portion, and reflected by the center gradient portion.
In the second photodetecting device, the second principal surface in opposing relation to the first principal surface of the semiconductor substrate having the light-receiving portion formed thereon is provided with the side gradient portion having the exposed surface located at the side portion thereof and with the center gradient portion having the exposed surface located at the near center portion thereof. Accordingly, even when the exposed surface of each of the gradient portions is provided with a (111) plane orientation, the light incident on the side portion is refracted by the side gradient portion and then reflected by the center gradient portion so that the direction of travel of the incident light is tilted by 6.3xc2x0 from a normal to the second principal surface. This further reduces the distance between the plane of the chip edge near the side gradient portion and the center portion of the light-receiving region as well as the chip size in a direction parallel to the principal surface.
In the second photodetecting device, the second principal surface preferably has a (001) plane orientation and the exposed surface of each of the side gradient portion and the center gradient portion preferably has a (112) plane orientation.
In the second photodetecting device, the second principal surface preferably has a (001) plane orientation and the exposed surface of each of the side gradient portion and the center gradient portion preferably has a (101) plane orientation.
In the second photodetecting device, the second principal surface preferably has a (001) plane orientation and the exposed surface of each of the side gradient portion and the center gradient portion preferably has a (111) plane orientation.
A third photodetecting device according to the present invention comprises: a semiconductor substrate and a light-receiving portion formed on a first principal surface of the semiconductor substrate, the semiconductor substrate having: a gradient portion formed by partially removing a second principal surface in opposing relation to the first principal surface; and an alignment mark formed on the second principal surface to be used in aligning the semiconductor substrate with respect to a mount on which the semiconductor is to be mounted, the light-receiving portion receiving light incident on a side portion of the semiconductor substrate and refracted or reflected by the gradient portion.
In the third photodetecting device, the second principal surface in opposing relation to the first principal surface of the semiconductor substrate having the light-receiving portion formed thereon is provided with the alignment mark for use in aligning the semiconductor substrate with the mount on which the semiconductor substrate is to be mounted. As a result, the light-receiving portion can be aligned with the alignment mark based on the accuracy of photolithography. This improves the accuracy with which the position at which the light is incident is controlled and increases the efficiency with which the incident light is coupled to the light-receiving portion.
In the third photodetecting device, the alignment mark is preferably formed by etching the second principal surface. This allows the formation of the alignment mark during the etching of the gradient portion and obviates the necessity for an additional process step.
Preferably, the third photodetecting device further comprises an electrode formed on the second principal surface of the semiconductor substrate, wherein the alignment mark is composed of the same material composing the electrode. This allows the formation of the alignment mark during the formation of the electrode on the second principal surface and obviates the necessity for an additional process step.
A fourth photodetecting device according to the present invention comprises: a semiconductor substrate; and a light-receiving portion formed on a first principal surface of the semiconductor substrate, the semiconductor substrate having: a gradient portion formed by partially removing a second principal surface in opposing relation to the first principal surface; and a shield film formed on the gradient portion and having an aperture for allowing the passage of a part of light incident on a side portion of the semiconductor substrate, the light-receiving portion receiving the incident light partially refracted or reflected by the gradient portion.
In the fourth photodetecting device, the gradient portion provided in the second principal surface has the shield film with the aperture allowing the passage of only the light incident on the light-receiving portion. Consequently, the angle of refraction between the refracted incident light and a normal to the second principal surface can be controlled accurately if a specified crystal plane is used at the exposed surface of the gradient portion provided in the second principal surface. This prevents the incident light from reaching the portion of the semiconductor layer (light absorbing layer) other than the light-receiving portion as well as the occurrence of a tail current.
Preferably, the fourth photodetecting device further comprises an electrode formed on the second principal surface of the semiconductor substrate, wherein the shield film is composed of the same member composing the electrode. This allows the formation of the shield film during the formation of the electrode on the second principal surface and obviates the necessity for an additional process step.